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1 Our Community’s Place Among the Stars Think About It Page E69 As you stargaze, what do you notice about the stars? Do some stars appear brighter than others? Larger or smaller? What about their colors? Date Page 2 Activity 7 Our Community’s Place Among the Stars Investigate Part B Page E71 1a. What does the vertical axis represent? 1b. What does the horizontal axis represent? 1c. What is the temperature and luminosity of the sun? 1d. Put four more dots on the diagram labeled A through D to show the locations of stars that are: a. Hot and bright b. Hot and dim c. Cool and dim d. Cool and bright 2a. Plot the locations of the stars from Table 1. 3. Classify each of the stars. Date Page 3 Star Sirius A Arcturus Vega Capella Rigel Procyon A Betelgeuse Altair Aldebaran Spica Pollux Deneb Procyon B Sirius B Type of Star Main sequence 4 Activity 7 Digging Deeper Pages E73-77 Stars Date Page vary in temperature and brightness Star’s classification depends on color and temperature of the star Luminosity describes the brightness of a star HR diagram a diagram that shows the relationship between a star’s brightness and temperature Temperature is plotted across the bottom, and luminosity is plotted up one side 5 Birth of a star 1. formation of a cloud of gas and dust 2. the material in the cloud is clumped, mix, and swirled 3. the core begins to heat up 4. when the temperature reaches 15 million Kelvins, fusion reactions begin 5. the energy released spreads into space, and a star is born 6 Fusion four hydrogen atoms fuse together to make one helium atom in a star’s core Some mass is “lost” in the reaction, because one helium atom has less mass than four hydrogen atoms This “lost” mass is changed into energy that is released by the sun http://www.youtube.com/watch?v=EO9CPO3CBF0 http://www.atomicarchive.com/Fusion/FusionMov.shtml Main sequence a star that uses hydrogen as its fuel 7 Main sequence stars fit into a diagonal band that runs from the upper left to the lower right of the H-R diagram Many stars spend 90% of their lifetime on the main sequence Our sun has a main sequence life span of about 10 billion years (it is now 5 billion years old) Star’s lifespan depend on its mass Example •our sun will live about 10 billion years •small, cooler stars live twice as long •massive, supergiant stars consume their mass too quickly only live a few tens of million of years •very hot stars go through their fuel very quickly 8 Hydrogen depletion when the hydrogen in the star runs out, there is no longer a balance between pressure (from fusion) and gravity The core contracts, and temperatures inside the star increase The outer layers of the star expand and cool The star now uses helium as fuel Giant (red giant) a star whose core has run out of hydrogen and is now using helium, causing its outer layers to expand and cool Giant stars are bright, but not hot (near the upper right corner of HR diagram) 9 http://www.youtube.com/watch?v=fOM7DMxOiAk 100 million K after the core of the giant reaches this temperature, helium fuses to form carbon The star expands to an enormous size, and its outer layers are much cooler Our sun will become a giant in about 5 billion years White dwarf a star that forms after much of the helium is used up, causing the star’s core to contract even more, and its outer layers escape into space 10 All that remains is the hot, dense core (about the size of Earth) White dwarfs are very hot, but not bright (lower left corner of HR diagram) http://whitedwarf.org/education/sunwd/index.html Massive stars in stars that are 8 times more massive than the sun, the stages of evolution are faster and more violent Supergiant a late stage in the life cycle of a massive star, when the core heats to higher temperatures, and heavier elements form by fusion The star continues to expand 11 Iron forms in the core, and fusion can no longer occur The core collapses violently and a shockwave travels outward through the star Supernova the outer portion of the star explodes, producing a supernova http://chandra.harvard.edu/resources/animations/sn_explosi on_lg_web.mpg http://www.nasa.gov/mpg/69478main_classic_supernova.m pg 12 Neutron star the collapsed, dense core of a supernova that contains only neutrons Stellar black hole the leftover core of a massive star after a supernova Black holes create such a large gravitational pull that not even light can escape 13 http://www.windows.ucar.edu/cool_stuff/movies/black_hol e.qt 14 Our Community’s Place Among the Stars Check Your Understanding Page E77 1. How do astronomers classify stars? 2. Write a brief outline of how stars are born. 3. What determines the way a star dies? Date Page